1. Field of the Invention
This invention pertains generally to pixelated detector imaging circuits, and more particularly to a method and apparatus for selecting an input channel with the highest signal amplitude from an array of detector input channels.
2. Description of the Background Art
Multi-channel solid state readouts are widely used in image detection applications where each pixel in a detector array is sensed independently from one another. Such readouts typically receive inputs from detector arrays and process the inputs to identify and selectively pass an analog signal corresponding to the highest amplitude input signal received. Within these systems the identification of a single cell containing the signal is resolved despite being constrained with signal levels which are not significantly higher than the noise level, and the existence of spurious signals within the other detector cells.
For example, a multichannel readout is commonly used in connection with a pixelated gamma camera which detects 140 keV photons resulting from positron annihilation. Such a detector couples a photodiode array to an array of scintillator crystals. Gamma ray interaction within one crystal of the scintillator array causes the appropriate cell of the photodiode array to produce a signal which is measured to determine in which crystal cell the interaction occurred and whether the energy deposited in that cell was consistent with a 140 keV photon.
Rapid sensing of the interaction within the scintillator array is required so that events are correctly registered and not missed. One accepted approach to providing for rapid sensing has been to feed the output from each detector cell into a multiplexer and a “winner-take-all” (WTA) circuit that identifies the highest amplitude signal and activates the multiplexer to pass that signal to the output. Since the device reads the detector cells in parallel to determine the highest amplitude cell, it has the capability of providing faster cycle times than a system which reads detector outputs serially.
Unfortunately creating a workable integrated circuit that can receive inputs from more than approximately sixteen detectors has posed a challenge. Mixing analog and digital circuitry within an IC layout causes moderate to severe noise problems that reduce the accuracy of the results and create erratic switching responses. In addition, the incorporation of a large multi-channel solid-state detector readout into a system brings about serious difficulties relating to control and testing issues.
Those skilled in the art will appreciate, therefore, that mixing analog and digital circuitry in an integrated circuit created noise or coupling problems that are very difficult to solve. While noise reduction has traditionally been addressed by careful circuit layout, there is a large penalty for failure due to the time and expense involved in integrated circuit design. Therefore, a need exists for a multi-channel solid-state detector readout circuit that allows parallel reading of the detector cells and overcomes noise, erratic response, and integration problems. The present invention satisfies those needs, as well as others, and overcomes the deficiencies of previously developed solutions.